At present, aimer has been widely used in the fields of firearms, bows and crossbows. Some auxiliary tools mounted on the firearm by holder are often required to be loaded when using the aimer. For example, in use of a red dot sight, a zoom lens is needed to amplify a target for a long distance and has to be dismounted for a short distance, thus the holder has to be mounted and demounted repeatedly. Besides, the operation is troublesome and inconvenient, thereby causing big inconvenience to the users.